1. Field of the Invention
The present invention relates to an inverter control device incorporating an AD converter, and an inverter control method.
2. Description of the Background Art
FIGS. 15A and 15B are illustrations each showing a configuration of a conventional inverter control microcomputer. An inverter control microcomputer 101 as shown in FIG. 15A includes a CPU 11, a ROM 12, a RAM 13, a timer 14, a serial port 15, a general-purpose I/O port 16, an inverter control signal generation circuit 17, and an AD converter 121. The inverter control microcomputer 101 is used as an element incorporated in a motor control device, which will be described below (see FIG. 2).
Conventionally, in order to conserve energy, a method for converting a position detection signal (analog signal) to a digital value by the AD converter incorporated in the inverter control microcomputer is adopted in the motor control device. In this method, an analog position detection signal is inputted to the inverter control microcomputer, and the AD converter incorporated in the inverter control microcomputer converts the inputted analog signal to a digital value. Next, the CPU incorporated in the inverter control microcomputer calculates a position of the rotor of the motor based on the obtained digital value. Next, the inverter control signal generation circuit incorporated in the inverter control microcomputer generates a motor control signal based on the obtained position information. The motor is controlled by the control signal generated as described above.
Here, in the case where a three-phase motor is used, it is preferable that two analog signals be used as position detection signals. However, the inverter control microcomputer 101 as shown in FIG. 15A has only one AD converter 121, whereby AD conversion cannot be performed concurrently for two analog signals. As a result, in the inverter control microcomputer 101, two analog signals are detected at slightly shifted time points. However, if there is a time lag between detection times of two analog signals, it is impossible to perform high-precision motor control because the motor is always running.
In order to solve the above-described problem, as shown in FIG. 15B, an inverter control microcomputer 102 having two AD converters 121 and 122 has been put into practical use. The inverter control microcomputer 102 can perform AD conversion concurrently for two-phase current values (U-phase motor current Iu and V-phase motor current Iv). Thus, it is possible to perform motor control with high precision.
Also, along with the enforcement of amendments to the Laws concerning the Rational Use of Energy and regulations on mains harmonics, a method for performing motor inverter control using a factor other than two-phase current values has been studied. Specifically, a method for enhancing the efficiency of motor inverter control by detecting a motor voltage and a power supply voltage in addition to motor currents has been studied. The Applicant of the present application has already proposed a semiconductor device for performing inverter control provided with more than two AD converters in Japanese Laid-Open Patent Publication No. 2002-165476 gazette.
For example, assume that the semiconductor device as described in the above gazette is used for detecting two-phase motor currents (U-phase motor current Iu and V-phase motor current Iv) and two-phase motor voltages (U-phase motor voltage Vu and V-phase motor voltage Vv). In this case, as shown in FIG. 16, by performing analog signal selection using two multiplexers 131 and 132 for selecting one output signal from among two input signals, it is possible to concurrently detect two-phase motor currents or two-phase motor voltages. However, the AD converters 121 and 122 need a predetermined time to perform AD conversion. Thus, in the configuration as shown in FIG. 16, it is impossible, for example, to concurrently detect the U-phase motor current Iu and the U-phase motor voltage Vu, and it is necessary to detect the U-phase motor voltage Vu after detecting the U-phase motor current Iu.
The easiest method of solving the above problem is to incorporate more AD converters into the inverter control microcomputer. For example, as shown in FIG. 17, it is possible to perform AD conversion concurrently for two-phase motor currents and two-phase motor voltages by separately inputting the two-phase motor currents and the two-phase motor voltages to different AD converters 121 to 124 via respective multiplexers 133 to 136. However, the configuration as shown in FIG. 17 has a drawback in that a cost of the inverter control microcomputer is increased with an increase in the number of AD converters.